1. Field of the Invention
This invention relates to composite electrodes for use with solid-state ionic devices. More particularly, this invention relates to composite electrodes for use with solid oxide fuel cells. More particularly yet, this invention relates to composite cathode electrodes for use with solid oxide fuel cells.
2. Description of Related Art
Solid state ionic devices typically consist of a fully dense electrolyte sandwiched between thin electrode layers. It is well known that the principal losses in most solid state ionic devices occur in the electrodes or the electrode/electrolyte interfaces. Thus, minimization of these losses is critical to efficient operation of these devices.
A solid oxide fuel cell is a solid electrochemical cell comprising a solid gas-impervious electrolyte sandwiched between a porous anode and porous cathode. Oxygen is transported through the cathode to the cathode/electrolyte interface where it is reduced to oxygen ions, which migrate through the electrolyte to the anode. At the anode, the ionic oxygen reacts with fuels such as hydrogen or methane and releases electrons. The electrons travel back to the cathode through an external circuit to generate electric power.
Conventional solid oxide fuel cells, although operable at temperatures in the range of about 600° C. to about 1000° C., generally exhibit high performance at operating temperatures only in the range of about 800° C. to about 1000° C. However, operation at such high temperatures causes physical or chemical degradation of the fuel cell construction materials. Thus, reducing the operating temperature of a solid oxide fuel cell to reduce such physical or chemical degradation and still maintain high performance levels is highly desirable. However, at reduced operating temperatures, e.g. 700° C., electrode reaction rates of conventional solid oxide fuel cells decrease significantly, substantially reducing cell performance.
It is well known to provide activated components on and/or within the fuel cell electrodes to support the electrochemical process. On the anode side, nickel is commonly used as a catalyst for oxidation of the fuel. On the cathode side, ceramic cathode materials, such as perovskites, typically employed in solid oxide fuel cells have a high activation energy for oxygen reduction. The activation energy for the oxygen reduction reaction may be reduced by adding noble metals such as Au, Ag, Pt, Pd, Ir, Ru, and other metals or alloys of the Pt group.
Efforts to increase electrode reactivity at lower temperatures have focused on optimizing the electrode microstructure and by introducing catalytic materials into the electrode structure. One such effort has resulted in the development of the electrode described and claimed in U.S. Pat. No. 6,420,064 B1 to Ghosh et al., the teachings of which are incorporated by reference herein in their entirety. The Ghosh et al. patent teaches an electrode for a solid oxide fuel cell, which electrode comprises a porous three-dimensional solid phase comprising an electrocatalytic phase comprising a plurality of electrocatalytic particles and an ionic conducting phase comprising a plurality of ionic conducting particles, wherein the phases are interspersed, and the mean or median size of the electrocatalytic particles is substantially equal to or larger than the mean or median size of the ionic conducting particles. In accordance with one embodiment, the electrode is a cathode comprising palladium (Pd) and yttria-stabilized zirconia (YSZ). This cathode is said to significantly improve the cell performance in the operating temperature range of about 725° C. to about 850° C. compared with conventional ceramic cathodes. However, at these temperatures, ferritic stainless steels that are low cost and commercially available have excessive corrosion rates that limit the solid oxide fuel cell device lifetime. Thus, it is desirable to be able to operate the cell at even lower temperatures, where corrosion rates are lower. In particular, operation in the 600° C. to 800° C. operating temperature range is desirable. Unfortunately, below about 700° C., the cathode has low electrochemical activity.